1. Field of the Invention
This invention relates to composite structures and more particularly although not exclusively carbon fibre composite structures utilising bolted attachments which are particularly susceptible to the injurious effects of lightning strikes. It is particularly described in the context of carbon fibre composite aircraft structures but is not exclusive to such an application.
2. Description of the Related Art
There is an inherent problem when lightning strikes attachment bolts in composite fibre structures whose heads are external to or countersunk within the outer structural layer, for example an aircraft wing skin. One important application is that of a carbon fibre wing structure, particularly a `wet wing` forming an integral fuel tank. In an adverse situation with a conventional nut and bolt attachment, lightning striking the bolt head, which would generally be flush countersunk into the wing outer surface, could result in arcing at the interface between the nut and bolt and local structure which could at the least cause degradation of the composite material but, in the worst case could ignite fuel vapour with catastrophic results.
One known arrangement of lightning resistant fastener is described in British Patent Application Number 2,212,580A. the fastener, used for joining an aircraft skin panel of composite material to a sub-structural member comprises a countersunk headed bolt and a capture nut wherein the bolt has a head surface region of an electrically conductive material for contacting the outer plies of the skin panel and a shank surface region coated with a layer of electrically insulting material for contacting the inner plies of skin panel and the substructural member, whereby lightning is conducted and dissipated controllably without arcing into the aircraft structure. It does not, however, provide passageway means for the passage of pressurized gases to atmosphere generated from the composite material when the assembly is subjected to lightning strike.
European Patent Application Number 248122 similarly describes a lightning protection system for conductive composite material structures and includes a fastener assembly in which the bolt is surrounded by and fits tightly within an expanded sleeve to provide good electrical contact between the bolt and the skin panel. In alternative embodiments the sleeve is either a one piece component extending over the depth of the structural assembly or two separate sleeve portions. This principle of intimate contact of the sleeves with both portions of composite structure differs from the present invention where the arrangement ensures non-conductivity to the inner structural element. Furthermore, this prior art arrangement makes no provision for the escape of pressurized gas to atmosphere generated when the composite material is subject to lightning strike. It is considered that this shortcoming increases the risk of the gases entering the volatile atmosphere in a fuel tank installation.